Keith Bradsher and Christopher Buckley

Sepang, Malaysia: The investigation into the baffling disappearance of Malaysia Airlines Flight 370 has been shrouded in even more confusion, with authorities backtracking on when a crucial communications system on the passenger jet was switched off.

Malaysian authorities have identified the plane's first officer, Fariq Abdul Hamid, as being the last person in the cockpit to speak to ground control before the flight vanished between Malaysian and Vietnamese airspace. The 27-year-old was recorded as saying calmly: "All right, good night."

However, the significance of that message is now once again being questioned after new comments disputing the timing of when the plane's communication system was switched off.

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Defence Minister Hishammuddin Hussein, who is also Malaysia's acting minister of transportation, appeared to give a crucial clue pointing to the possible complicity of the pilots when he said at a news conference on Sunday that the communications system had been "disabled" at 1.07am on March 8. That was before the verbal sign-off was given to air traffic controllers on the outskirts of Kuala Lumpur.

But Ahmad Jauhari Yahya, the Malaysia Airlines CEO, on Monday then contradicted that information, saying Hamid’s final voice transmission may have occurred before any of the jet's communications systems were disabled.

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He said the communications system, known as an Aircraft Communications Addressing and Reporting System, had worked normally at 1.07am but then failed to send its next regularly scheduled update at 1.37am.

The backflip overnight is the latest in a string of contradictory statements made by authorities as the investigation stretches into a 10th day.

The new uncertainty could raise additional questions about whether the plane was deliberately diverted or whether it suffered mechanical or electrical difficulties that crippled its communications and resulted in its flying an aberrant course that involved turning around, heading back over peninsular Malaysia while rising and falling rapidly again, and finally flying out over the Strait of Malacca to an unknown location.

Mr Ahmad Jauhari said it was between the two scheduled transmission times for the Acars system that the verbal signoff was given by radio at 1.19am.

A second communications system, a transponder that communicates with ground-based radar, then ceased working at 1.21am.

The new description of what happened to the Acars system appeared to reopen the possibility that the aircraft was operating normally until the transponder ceased sending signals two minutes after the last radio message.

Standing next to Mr Ahmad Jauhari, Mr Hishammuddin waved off numerous questions about why he had said a day earlier that Acars had been disabled at 1.07am.

"What I said yesterday was based on fact, corroborated and verified," he said. In response to another question, he said that uncertainty about the chronology underlined the importance of finding the aircraft and its data recorders.

The last satellite transmission from the Boeing 777-200 on March 8 may have come from over Indonesia or the southern Indian Ocean, Malaysian officials said. The alternative is that the transmission came from western or southwestern China, or from nearby areas of Kyrgyzstan, Kazakhstan or northern Laos.

Australia announced Monday that it would search the vast expanse of the southern Indian Ocean for the missing jetliner.

Prime Minister Tony Abbott of Australia spoke with Prime Minister Najib Razak of Malaysia on Monday afternoon and offered extra resources for the search, which now involves 26 nations.

"We are currently working on a defined search zone," the spokesman said, adding that Abbott was expected to release a more detailed statement on Monday evening.

Police have been investigating the pilot, co-pilot and other crew members of the missing Malaysia Airlines flight since the day of its disappearance, Malaysia's Transport Ministry said in a statement Monday. The statement highlighted growing interest by law enforcement authorities into whether any of the airline employees might have been complicit in the plane's disappearance.

Adam Dolnik, a professor at the University of Wollongong in Australia who has studied terrorism in Southeast Asia and other parts of the world, said that, judging from the information disclosed so far, there was no evidence to suggest involvement by a terrorist organisation, although there was the possibility of a "lone wolf" acting, at least partly, in the name of extremist beliefs.

Professor Dolnik voiced scepticism that the two Iranians on board the plane with stolen passports had played a role in diverting it.

"For groups like al-Qaeda, which tried to take airliners down in midcourse flight by a suicide bomber since the mid-1990s, this is their fantasy target and what they keep going for, but repeatedly they are unable to keep doing it," he said. "But for a group like this to grow an entire plan - which would have to be quite sophisticated for them to be able to actually get the operational capabilities through the secure perimeter and onboard an airplane - and to blow it on something like a stolen passport, it just doesn't make any sense. What they would do is send operatives who have clean passports, to make sure they actually make it through immigration."

Mr Dolnik added: "If you're a militant jihadist group, why would you ever go for Malaysia Airlines? If you have a predominantly Muslim country, one of the biggest Muslim countries, hitting the national carrier of that country really would be very risky in terms of constituency support or how people are going to view you."

Malaysia's Transport Ministry also said that three civil aviation security officials had arrived from France to share expertise gained from the search for Air France Flight 447, which disappeared nearly five years ago off the coast of Brazil. Searchers there needed almost two years to find the Air France jet, an Airbus A330, on the floor of the Atlantic Ocean.

Investigators had an advantage then because they had found more than 3000 pieces of floating debris and 50 bodies in the ocean in the days and weeks after the crash, giving them a rough sense of where the plane had entered the water. By contrast, there are still few clues regarding where Malaysia Airlines' Boeing 777 finally came down after someone diverted it 40 minutes into what was supposed to be a flight from Kuala Lumpur to Beijing.